The present invention relates to valve actuators and, more particularly, to electrically operated actuators for mounting on zone valves in heating systems.
Zone valves are installed in various places in lines carrying a heated fluid to space units in a structure to be heated in order to control the flow of the heated fluid to the space units in response to the temperature in the space. In many instances, the zone valves are controlled by electric valve actuators which are triggered by thermostats in order to open or close individual zone valves and, thereby, initiate or terminate the flow of a heated fluid to the space units associated with a particular zone valve and thermostat. Zone valves ordinarily have a reciprocative valve element which is biased to a closed position against its seat by a spring. The valve actuators have a thermostatic device including a reciprocating thrust stem which engages the valve stem and is capable of forcing the valve element open by the operation of a thermostatic element containing a wax or other material which expands upon the application of heat. Typically, when a thermostat senses a need for additional heat, it completes a circuit energizing a heating device, for example, an electric resistance heating element, positioned in heat exchange relationship with the thermostatic element. The heating device heats the thermostatic element so that the thermostatic element expands, overcoming the bias of the valve spring and opening the valve.
Such known actuators have been known to encounter difficulties if an obstruction occurs on the seat of the zone valve or below the seat element. The obstruction can prevent the valve from fully closing or opening and, in either case, can cause electricity to continue to flow to the electric resistance heating element, which in turn causes the thermostatic element to overheat and fail, usually by breaking through the top of a cup containing the thermostatic element. For this reason, filters have been provided for the zone valves to prevent dirt or an obstruction from accumulating on the valve seat or lodging below the seat element. In addition, the electric resistance heating element usually comprises a bare resistance wire wound around the cup and, as a result, leaking wax from a failed thermostatic element has been known to flow down from the top of the cup onto the electric resistance element and catch fire from the heat generated by the element. The problem is compounded in some cases by the use of a flammable plastic cover for the actuator, which fails to contain the heat and can add fuel to the fire rather than contain it. Such covers are usually open to permit heat to dissipate, but the openings permit moisture to enter and frequent temperature changes to occur, which prematurely age rubber elements in the thermostatic elements. Furthermore, the actuators according to the prior art employ open switches in the actuator housing, which are not dust-tight. Consequently, the actuators have failed to operate in dirty environments due to dirt preventing the closing of exposed switch contacts. Moreover, since the thermostatic elements remain hot and exert a force on the thrust stem for some time after they are heated, the thrust stems have been known to shoot out of an actuator which was being removed from the valve by a workman and cause injury.
Manual valve actuators associated with known power actuators directly engage the valve stem and will cause the thermostatic element to overheat and fail if power operation of the actuator is restored while the manual actuator holds the valve open.